Sunday, November 20, 2011

Acronym in astronomy: TNO for trans-Neptunian object

Since the deomotion of Pluto to dwarf planet status, Neptune is the outermost planet of our solar system. A body orbiting the sun beyond Neptune's orbit is called a trans-Neptunian object, TNO for short.  For example, Eris, Pluto (!),  2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar and Varuna are TNOs (shown to scale and in reference to the size of Earth in Figure 4.9 in [1]). Many more are known—classified, numbered, but often without a common name [2,3].

Often, TNOs come in pairs, called binaries:  two bodies with similar masses that orbit each other [4]. Pluto and its moon Charon are such a couple [5].

TNOs are not planets. Three TNOs are called dwarf planets: Pluto, Eris, and Makemake [6]. Ceres also is a dwarf planet, but not a TNO. Ceres belongs to the asteroid belt between Mars and Jupiter.

The use of the prefix trans (typically meaning through) in the term trans-Neptunian has occasionally been critized, since, what really is meant is ultra in the sense of  beyond. However, then we would get the term ultra-Neptunian object and the acronym UNO. The latter stands for United Nations Organization, concerned primarily with one object—our home planet.

Keywords: astronomy, planetary science, solar system, celestial bodies, terminology.

References and more to explore
[1] Neil deGrasse Tyson: The Pluto Files. W. W. Norton & Company, New York and London, 2009; pages 67, 91 and others.
[2] IAU Minor Planet Center > List Of Transneptunian Objects:
[3] List of Known Trans-Neptunian Objects:
[4] The Planetary Society > Explore > Space Topics -Trans-Neptunian Objects:
[5] 30 Years Since Charon Reveals Pluto To Be A Binary Planet System:
[6] IAU > Fourth dwarf planet named Makemake

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